I find the pull bouy useful for a session of drills, when the aim is to slow the pace right down, focus on the arm techniques, breathing or stroke timing. After half a dozen lengths you ditch the toy and try to put in into practice on your ownio and swim the rest of your session until the onset of fatigue.
*** This is pretty much a description of our coached session.
Midweek I'm just going in and doing endurance training. 1.5k, 2k, 2.5k at the moment, depending on available time.